I'm not a civil engineer, but I think the process is something like this.

You need to determine your flow and pressure requirements at the most remote location on your system. Once you know that, you need to create a pressure maintenance system, such as a dam, a water tower/tank/pump, or a well and pump, to deliver that flow and pressure plus allowable pressure drop through the water mains. You also need to determine how much water capacity you will need from your source be it well, or dam, or other. If it is to be a combined fire and domestic water system you will need to account for both potentials occurring in part simultaneously.

Example flow and pressure may be 50 psi flowing 1200 gpm at the most remote location.